Annals of Botany 80 : 289–297, 1997 Energetic Costs of Tissue Construction in Yellow-poplar and White Oak Trees Exposed to Long-term CO 2 Enrichment STAN D.WULLSCHLEGER*, R.J.NORBY, J.C.LOVE and C.RUNCK Enironmental Sciences Diision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6422, USA Received : 12 July 1996 Accepted : 2 April 1997 Two methods were used to estimate construction costs for leaves, stems, branches and woody roots of yellow-poplar (Liriodendron tulipifera L.) trees grown at ambient (35 Pa) and elevated (65 Pa) CO  for 27 years and trees of white oak (Quercus alba L.) grown at these same CO  partial pressures for 4 years. Sample combustion in a bomb calorimeter combined with measurements of ash and nitrogen content provided the primary method of estimating tissue construction costs (W G ; g glucose g - dry mass). These values were compared with a second, simpler method in which cost estimates were derived from tissue ash, carbon and nitrogen content (V G ). Estimates of W G were lower for leaves, branches and roots of yellow-poplar and for leaves of white oak grown at elevated compared with ambient CO  partial pressures. These CO  -induced differences in W G ranged from 37 % in yellow-poplar roots to 21% in white oak leaves. Only in the case of yellow-poplar leaves, however, were differences in V G observed between CO  treatments. Leaf V G was 146 g glucose g - dry mass in ambient-grown trees compared with 141 g glucose g - dry mass for CO  -enriched trees. Although paired-estimates of W G and V G clustered about a 1:1 line for leaves and branches, estimates of V G were consistently lower than W G for stems and roots. Construction costs per unit leaf area were 95 g glucose m - for yellow-poplar trees grown at ambient CO  and 106 g glucose m - for trees grown at elevated CO  partial pressures. No differences in area-based construction costs were observed for white oak. Whole-plant energy content was 1220 g glucose per tree in ambient-grown white oak compared with 2840 g glucose per tree for those grown at elevated CO  partial pressures. These differences were driven largely by CO  -induced changes in total biomass. We conclude that while construction costs were lower at elevated CO  partial pressures, the magnitude of this response argues against an increased efficiency of carbon use in the growth processes of trees exposed to CO  enrichment.  1997 Annals of Botany Company Key words : Bomb calorimeter, construction costs, elevated CO  , energy allocation, global change, growth respiration, heat of combustion, respiration, Liriodendron tulipifera, Quercus alba. INTRODUCTION Among the many plant physiological processes known to be affected by atmospheric CO  enrichment, one of the more controversial is the varied response of leaf and whole-plant respiration to elevated CO  partial pressures (Poorter et al., 1992). Such a controversy has been fuelled by reports that respiration may increase (Thomas et al., 1993 ; Thomas and Griffin, 1994), decrease (Amthor, Koch and Bloom, 1992 ; Ziska and Bunce, 1993 ; Bunce, 1995 ; Teskey, 1995), or remain unchanged (Ryle, Powell and Tewson, 1992 ; Mitchell et al., 1995) in response to both short- and long- term CO  exposure. Although these and earlier reports (see reviews by Amthor, 1991 ; Wullschleger, Ziska and Bunce, 1994) have generated much debate, it is agreed that since a large portion of the carbon fixed by plants through photosynthesis is returned to the atmosphere via respiratory processes (Post et al., 1990), establishing the likely direction and magnitude (if any) by which respiration changes in response to elevated CO  should be an important goal of plant response studies to global change. Attempts to better quantify the direct andor indirect respiratory response of plants to elevated CO  have led to * For correspondence. an expanded study of respiration, with consideration given not only to the fundamental processes involved in these responses (Azco  n-Bieto et al., 1994 ; Reuveni, Mayer and Gale, 1995), but also to how the individual components of growth and maintenance respiration are affected by atmos- pheric CO  enrichment. Thomas et al. (1993) measured respiration during leaf expansion in Gossypium hirsutum L. grown at 35 and 65 Pa CO  and noted that while growth coefficients were similar between treatments, rates of maintenance respiration were higher at elevated CO  partial pressures. Amthor (1995) has discussed how this higher maintenance respiration can be explained by greater respiratory costs of carbohydrate translocation from CO  - enriched leaves. More recent studies with Glycine max (L.) Merr. have observed higher (Thomas and Griffin, 1994) and lower (Bunce, 1995 ; Bunce and Ziska, 1996) rates of maintenance respiration for plants grown at elevated CO  partial pressures, yet both studies report that growth coefficients did not change in response to CO  enrichment. Earlier studies with trees indicated that maintenance respiration was lower in CO  -enriched leaves of Liriodendron tulipifera L. (Wullschleger, Norby and Gunderson, 1992 a) and that both leaf maintenance and growth respiration were lower in Quercus alba L. grown at elevated compared with ambient CO  (Wullschleger and Norby, 1992). Lower leaf 0305-73649709028909 $25.000 bo970434  1997 Annals of Botany Company